Electronic power converters are used extensively for modifying readily available sources of electric power to suit specific applications. These converters are used heavily in the manufacturing industry, in conventional and renewable power generation and transmission, and in automotive applications. Such converters may provide a direct current (DC) to DC function, a DC to alternating current (AC) function or an AC to AC function. These converters use solid-state devices, such as power semiconductor switches, typically insulated gate bipolar junction transistors (IGBTs) and metal oxide semiconductor field effect transistors (MOSFETs), to achieve the required switching process. While the switches enable the transfer of great amounts of power, their control, essentially the turn on and turn off functions, is accomplished using very little energy through a gate terminal.
Desaturation protection circuits monitors switch voltage while the switch conducts current. In the case current becomes excessive, the protection circuit would detect enlarged switch voltage and trigger the safe turn off. The turn off process results in large voltage overshoot across the main terminals. This excess voltage could destroy the device under unfavorable conditions, specifically an excessive current situation. One common way of detecting such an impending destruction is to observe the desaturation of the IGBT when the voltage across the collector-emitter terminals rises well above the desired, saturated condition. The IGBT is turned off immediately upon detection of desaturation through a slower than normal process to limit the voltage overshoot. The desaturation detection circuit includes a ‘blanking’ time to prevent premature reaction and possibly a level adjusting feature.
Electric motor drive systems as generally employed in electric vehicle applications typically include an AC electric motor, a power inverter and a DC power source, such as a storage battery. The power inverter typically converts the DC electric power from the power source to AC electric power to drive the electric motor. The inverter may include IGBT or similar power semiconductor based switches that switch in a complementary manner to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives one or more wheels of the vehicle thereby providing propulsion.
When the IGBT of an IGBT-based inverter is fully on, it provides a low resistance, although not resistance free, conduction path from the collector to the emitter of the IGBT. The fully on voltage from collector to emitter is referred to as the saturation voltage, Vcesat. If the current flow through the IGBT increases above a permissible threshold, the collector-emitter voltage rises above Vcesat, and indicates the IGBT is no longer operating in the saturation region, i.e., it has become desaturated. Continued operation in desaturation can lead to failure of the IGBT, and hence the circuit designer is challenged to design a gate driver circuit that will turn the IGBT off when desaturation is detected. Advantageously, desaturation can be detected by detecting the rises of the collector-emitter voltage above Vcesat. Commercially available IGBT gate driver integrated circuits (ICs) are designed to detect a desaturation voltage, Vcedesat, above a fixed threshold before taking action to turn the IGBT off. Providing a fixed threshold within the IGBT gate driver circuit limits the ability of the inverter designer to adjust the desaturation threshold to a given application to take full advantage of the IGBT capabilities.